Digital control is becoming more and more popular because of the process advancement of CMOS technology. In addition, the power requirement is becoming more and more complicated and the conventional analog control method can no longer meet this requirements. Digital control of power converter and power system can meet these more and more complicated power requirement. It is expected that digital control will replace analog control in switching power converters.
At the present time, the control algorithms used in digital controllers are translated directly from analog algorithms. In other words, they are digital implementation of the algorithms that are optimized for analog implementation. They are not optimized for digital implementation.
For DC-DC converter, current mode control is widely used because of its advantages. Peak current mode control, average current mode control, and hysteresis current mode control are three commonly used current mode control methods.
Current mode control can improve the dynamic performance of the DC-DC converter significantly because it utilizes the inductor current information, together with the output voltage information. Unfortunately, the conventional current mode control methods are optimized for analog implementation. It is very difficult to implement those methods using digital circuits.
Thus, there is a need for a new current mode control method that is suitable for both analog and digital implementation. It is called parallel current mode control. The parallel current mode control can be applied to both DC-DC converter and AC-DC converter with power factor correction.
There are several disadvantages in existing digital control PFC implementation systems based on conventional current mode control, such as high computational requirement, limited switching frequency and high cost. Predictive control methods are being explored and implemented in digital controlled PFC in order to take full advantage of digital techniques. One such digital current program control using a predictive algorithm was presented by Chen et al in “Predictive Digital Current programmed Control”, IEEE Transactions on Power Electronics, Vol 18, No 1, January 2003, pp 411-419. In that paper, the duty cycle, d(n+1), was calculated based on the value of the present duty cycle d(n) and sensed inductor current, input voltage and output voltage. Unity power factor was achieved. The first disadvantage is that the duty cycle calculation requires the duty cycle value in the previous switching cycle. Second, the computation requirement is not obviously reduced compared to that in the digital PFC implementation based on current mode control. Bibian et al, in “Digital Control with improved performance for boost power factor correction circuits”, Applied Power Electronics Conference and Exposition 2001, 16th Annual IEEE, pp 137-143 proposed dead-beat predictive control in which a predicted duty cycle was used to control the switch during a control period which is equivalent to several or several tens switching cycles. The duty cycles were fixed during one control period. Computation was reduced in that control method. However, the harmonics in the line current was increased compared to the control method in which the duty cycle was calculated in every switching cycle. The computation requirement in digital PFC implementations was reduced further by the techniques proposed by Zhang et al in APEC 2003, pp 403-409 and PESC 2003, pp 335-341 because all the duty cycles for a half line period were calculated in advance based on the voltage loop and the input voltage feed-forward. However, the current waveform is sensitive to the parameters of the model and the capability of the regulation to the step load change is not satisfied when the load variation is wide. Thus there is still a need for a direct duty cycle control algorithm with low calculation requirement for digital power factor correction implementation.